The long-term goals of this research are to understand the neural basis of how memories acquired during song learning are stored in the songbird brain; how these memories interact with auditory experience and the influence of steroid hormones to modulate the response properties of auditory neurons; and how these response properties reflect or determine what songs a bird will sing and how it will respond behaviorally to song. Bird song acquisition is probably the closest animal analog of human speech development. Specialized neural structures have evolved to subserve both behaviors, and both speech and song are most easily learned during an early impressionable period and require species-typical auditory input and auditory feedback. Furthermore, in both instances, normal perceptual development may require normal motor acquisition. The development of both speech and song is impaired by abnormal sensory experience, such as that caused by temporary conductive hearing loss in children or when a young bird is prevented from hearing appropriate song models or from hearing its own vocalizations. Further understanding of the neural basis of song learning may provide a model of how the sensory input required for specialized behaviors is stored, accessed, and used in motor and perceptual development, and it may also uncover the cellular mechanisms that cause abnormal early sensory experience to limit behavioral development. The proposed research focuses on the song-control nucleus HVc. This nucleus is necessary for song production in adult birds, and auditory/motor interactions within it are probably important during song acquisition. The specific aims research are: 1) to determine the anatomical locus of auditory input to HVc; 2) to determine the internal circuitry and projections of sub-classes of neurons within this nucleus; and 3) to investigate further how sensory experience, song production, and steroid hormones interact in the development of auditory response properties of neurons in HVc. The experiments combine anatomical and electrophysiological investigations of nucleus HVc, and of its inputs and outputs. Extracellularly applied neuronal tracers will be used to determine the source, development, and organization of auditory inputs to HVc. Intracellular recording and dye-filling will then be used to find out how different populations of neurons in HVc connect with each other and with cells in other song-system nuclei. Methods will be developed to prevent a bird from hearing itself sing during the normal period of song acquisition, and the development of song and of selective auditory responses in HVc will be monitored behaviorally and electrophysiologically in these birds.